The only thing those "performance chips" void is the purchaser's credibility with regards to their spending habits.
The only modern-day vehicles I've seen benefit from any kind of "chipping" are those with stock turbos...and then those chips are in the $200-500 range.
A buddy and I went to the local drag strip one day after purchasing one of those eBay specials. With the "performance chip" installed we were neck-and-neck. When he put his stock chip back in he was well over two car lengths ahead of me (as is befitting of a Pontiac G6 vs a 4-banger Altima Coupe). I think he spent a total of $50 on the stupid thing.
Just remember...power ain't cheap.
On that note, I used to run a BullyDog on the aforementioned Altima Coupe. It allowed me to advance the timing by two degrees, raise the rev limiter, and kaibosh the governor. It also cost me $300. Honestly not worth it for a limited-by-design 4 cylinder engine.
With regards to voiding warranties, I was told by the Nissan dealership that as long as the aftermarket parts could be removed, they wouldn't void anything so long as they weren't the cause of any problem. The exception to this was anything that changed the stock computer programming. I'm sure Toyota takes a similar stance...